Currently used blue pigments are Cobalt blue (CoAl2O4), ultramarine (Na7Al6Si6O24S3), Prussian blue (Fe4[Fe(CN)6]3), and Azurite [Cu3(CO3)2(OH)2]. All suffer from environmental and/or durability issues: Cobalt is considered to be highly toxic. Ultramarine and azurite are not stable with respect to heat and acidic conditions. Prussian blue liberates HCN under mild acidic conditions. In addition, the manufacture of ultramarine involves a large amount of SO2 emission. Hence, the identification of intense blue inorganic pigments that are environmentally benign, earth-abundant, and durable is important but remains a challenge today.
Barium and copper silicates, especially of blue or violet colour, are furthermore known, however the preparation of these products presents some difficulties. In fact, these silicates are generally obtained by grog firing. In such cases, because of the high temperatures required for the preparation, melt phases are necessarily involved as intermediates and as a result products are obtained essentially in the form of glasses [Reference may be made to Andrew E. Smith et al. J. Am. Chem. Soc. 2009, 131, 17084-17086].
Reference may be made to U.S. Pat. No. 5,888,291, wherein, the inventors have claimed a process for the preparation of blue/violet inorganic pigments based on barium, copper and titanium silicates, represented by the formulae: BaCuSi4O10, BaCuSi2O6 and BaCuTiSi3O9, wherein said silicate comprises agglomerate with an average particle size of no greater than 20 micrometers. These silicates are prepared by a process including the following stages: a silica sol or silicate and optionally a titanium sol is mixed with sols of the other constituent elements of the silicate with the exception of oxygen; the mixtures thus obtained is dried, preferably by spraying, and the product thus obtained is calcined. This patent also discloses a process for the preparation of a series of rare earth doped pigments, especially lanthanum and neodymium having the formulae: Ba0.5La0.5Cu0.5Na0.5Si4O10, Ba0.5Nd0.5Cu0.5Si2O6 and Ba0.5La0.5Cu0.5Na0.5Si2O6, well suited for colouration of plastics, paints, varnishes, rubbers, ceramics, glasses, inks, cosmetic products and dyes. However, the pigment powder silicates obtained contains mixture of phases. The main drawbacks with these pigment powders is that they are not stable and light-green colours were observed when they were immersed in the unleaded commercial transparent glaze. Most importantly, the colour coordinates obtained are not very attractive [Ba0.5La0.5Cu0.5Na0.5Si4O10: L*=58.0, a*=5.0, b*=−43.0] as compared to present invention [La0.3Sr0.7Cu0.7Li0.3Si4O10, L*=49.59, a*=10.41, b*=−51.74].
Solid solutions of CaCuSi4O10, BaCuSi4O10 and SrCuSi4O10 have been successfully synthesized from stoichiometric mixtures of silica, alkaline-earth metal carbonates and copper carbonate at high temperature using solid state reactions by Kendrick and co-workers [Reference may be made to E. Kendrick, C. J. Kirk, S. E. Dann, Dyes and Pigments 73 (2007) 13-18]. UV-vis spectra and colour coordinate data were collected on the materials to correlate the changes in the structure with the observed colour properties. A concomitant reduction in lightness indicating potential weakening of the tinting capability is observed as the cell parameters lengthen. Prolonged exposure to natural light has a deleterious effect on the colour of these synthetic materials in a similar way to the natural minerals. Further, the blue of the pigment is not very attractive.
Mixed oxides such as Ba0.5La0.5Na0.5Cu0.5Si4O10 and Ba0.5La0.5Na0.5Cu0.5Si2O6 produced vivid blue and violet-blue colour powders, respectively when calcined between 950° C. and 1050° C. for different soaking times. X-ray diffraction patterns for Ba0.5La0.5Na0.5Cu0.5Si4O10 showed the presence of the phases which included (a) BaCuSi4O10 (b) La2Si2O7 (c) SiO2 and La2O3 (trace) whereas Ba0.5La0.5Na0.5Cu0.5Si2O6 confirmed the presence of the phases such as (a) BaCuSi4O10 and (b) BaCuSi2O6. These pigment powders were also not stable and light-green colours were observed when they were immersed in the unleaded commercial transparent glaze [Reference may be made to: Jitendra Kumar Kar, Ron Stevens, Christopher R. Bowen, Journal of Alloys and Compounds 461 (2008) 77-84]. Further, the colour coordinates are not very attractive [L*=65.58, a*=1.92, b*=−38.01] as compared to present invention [La0.3Sr0.7Cu0.7Li0.3Si4O10, L*=49.59, a*=10.41, b*=−51.74].
In order to obviate the drawbacks associated with the existing blue pigments, the present invention reports a phase pure silicate with impressive chromatic characteristics, in particular a remarkable blue colour with high near infra-red reflectance.